This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The genetic code is established by a system of 20 enzymes, aminoacyl-tRNA synthetases, which attach specific amino acids to tRNAs carrying anti-codon sequences cognate for the amino acid. Similarly, it has been recently proposed that the more numerous tRNA modification enzymes bind to their RNA substrates according to a complex recognition code. We want to understand the structural basis of highly specific tRNA recognition by these enzymes. A well studied synthetase is alanyl-tRNA synthetase which has been recently crystallographically deciphered (Swairjo et al., Mol. Cell., 2004, 13(6):829-841). This synthetase recognizes its cognate tRNA at a conserved G:U base pair in the acceptor stem, and is indifferent toward the tRNA anticodon. We have large crystals of AlaRS complexed with an RNA substrate mimicking the acceptor stem of tRNAAla. Similarly, YHDG and Ykvm are two novel enzymes involved in two different and specific tRNA modifications. We have crystals of Ykvm, and are currently persuing its tRNA complex with crystaliization trials.